1. Field of the Invention
This invention relates to VLSI technology and more particularly to improved VIA structures therein.
2. Description of Related Art
Metal VIA plugs are used to planarize VIA holes, in addition to connecting upper level metals and lower level metals. The current practice is to employ a tungsten plug, requiring a well controlled VIA hole profile, which requires a complex manufacturing process.
FIG. 1 shows a prior art device 10 with a tungsten via metal via plug 16 formed from the top down. Device 10 includes a silicon substrate 11, upon which conductor 13 is formed. A blanket dielectric layer 12 covers the substrate 11 and a portion of the conductor 13. Above the central portion pad 13 is a tungsten via plug 16 formed in an opening in dielectric layer 12. Before the plug 16 was formed a "glue" bonding layer 15 composed of TiN/Ti was blanket deposited over the dielectric layer 12. The plug 16 was formed by first forming a blanket deposit of a tungsten film on the bonding layer 15. Then a blanket etch back step, or a polish back step, is performed so that all of the tungsten is removed except in the via hole. After the tungsten plug 16 has been formed, a conductor layer 17 was formed composed of aluminum alloy/titanium.
FIG. 2 shows another prior art device 10 different from FIG. 1, in part, because of the substitution of aluminum metal plug 19 for tungsten metal via plug 16. The process by which the device of FIG. 2 is formed comprises the blanket deposition of a barrier metal layer 15 first, and then an aluminum metal layer was deposited at an elevated temperature so that the via hole is filled by aluminum metal. Alternatively, the wafer may be subjected to an elevated temperature treatment after aluminum deposition in order to make aluminum flow and fill the via hole. The major problem associated with this method is that the elevated temperature, &gt;400.degree. C. is very sensitive to cause device junction leakage.
Note that in Sze, VLSI Technology, McGraw-Hill Book Company, page 409, (1988) it is stated that "Diffusion barriers such as TiN, TiC, and Ti-W are found to be stable at 450.degree. C., and no metallurgical or electrical degradation of the silicide-barrier-aluminum contact metallization has been reported at 450.degree. C. as long as these barrier films are pinhole free. Generally a thickness in the range of 500-1000.ANG. is appropriate. Note that TiN and TiC are the compounds of nitrogen and carbon with titanium, respectively, whereas Ti-W is a deposited mixture of the two elements, which may form intermetallic compound or compounds depending on the heat treatment following the deposition."